Piers and Underpinning: Push Piers, Helical Piers and Concrete Piers
Underpinning means adding support under an existing foundation. Piers are the most common way: columns that reach deeper, more stable soil than the soil that moved. The details of the system matter less than whether the design fits your soil and whether the lift target is realistic.
What underpinning is
Underpinning extends support beneath an existing foundation to deeper or more stable ground. The ASCE Texas Section guidelines list it among remedies for slab foundations and caution that it generally does not improve non-underpinned areas, and that adjusting elevation is unlikely to produce a fully level foundation.[1]
Common pier types
The following are general descriptions; we found no independent comparative study of residential pier systems, and most freely available comparisons are published by repair companies that sell one system.
| Type | General description | Points to consider |
|---|---|---|
| Push (resistance) piers | Steel sections pushed hydraulically into the ground, using the weight of the structure as the reaction, until they reach resistance or bedrock, then used to lift. | Depth and capacity depend on how far they can be pushed against the structure's weight. Ask how capacity is determined. |
| Helical piers (screw piles) | Steel shafts with spiral plates that are screwed into the ground; capacity depends on plate diameter, pile length and soil. A builder's JLC article describes short bolted sections used in tight spaces, installed with a hydraulic rotating head to a depth set by an engineer's test pile.[2] | Needs about 4 feet of vertical clearance at each location in the JLC example, so excavation may be required in tight spaces. Depth should be confirmed by an engineer's test, not assumed.[2] |
| Poured or pressed concrete piers | Concrete cylinders or drilled shafts placed or pressed to a chosen depth, often tied to the foundation with grade beams. | The Colorado Geological Survey describes drilled piers extending well below the seasonal moisture zone and anchored in non-swelling soil for new construction on swelling soils.[3] |
What the engineering sources say about doing it right
- The pier design, including locations, spacing and depth, should come from an engineer or from a documented method, not from a count chosen to hit a sales price. In the JLC example an engineer set locations about 7 feet on center and a test pile determined the depth.[2]
- Lifting should be controlled and incremental. The American Concrete Institute's slabjacking guidance limits lift to 1/4 inch at one hole at a time to avoid cracking; piering has its own tolerances, so ask what is planned.[4]
- Underpinned areas do not make the rest of the foundation better.[1]
- Test water, sewer and gas lines after lifting.[1]
Questions to ask about any pier proposal
- Who designed it, and is the design stamped by a licensed engineer?
- How is the capacity of each pier determined, and how deep is it expected to go?
- How many piers and where, and why those locations? What is the lift target?
- What happens if a pier does not reach resistance at the planned depth?
- Is a post-repair elevation survey included?
- What does the warranty cover, for how long, and does it transfer to a new owner? See contracts, warranties and permits.
Cost and alternatives
Pier jobs are among the most expensive foundation repairs and the published price ranges vary widely; see what foundation repair costs. Alternatives or complements include drainage correction, grouting where appropriate, and monitoring when movement has stopped.
Common questions
What is the difference between push piers and helical piers?
In general terms, push piers are pushed into the ground using the structure's weight as reaction, while helical piers are screwed in using plates welded to the shaft, with capacity set by plate size, depth and soil.[2] Which is appropriate depends on soil and design.
How many piers does a house need?
It depends on the house and soil; the number should come from a design and elevation survey. Be skeptical of a count that is not explained.
Do piers permanently fix a foundation?
They can stabilize the supported areas, but they do not change the soil elsewhere, and the ASCE guidelines say underpinning generally does not improve non-underpinned areas.[1]
Can piers be installed in tight spaces?
Often, but equipment and clearance matter. In the JLC example, helical piles were installed with a skid steer-mounted drive head, needing roughly 4 feet of vertical clearance at each location.[2]
Related guides
Sources
Figures and claims marked with a number come from these sources. Dates are the publication or last-update dates shown by the publisher, where one was shown. Older sources are flagged in the text.
- Guidelines for the Evaluation and Repair of Residential Foundations, Version 1. Texas Section, American Society of Civil Engineers; adopted October 3, 2002, effective January 1, 2003.Written for Texas conditions and by its own terms not a standard of care. Widely used as a reference for slab and pier-and-beam evaluation. Published in 2002.
- Retrofitting a Foundation for Problem Soils. Journal of Light Construction; Nathaniel Eldon, builder; undated.A builder's account of one project, used here only to illustrate how helical piles are installed.
- Home Construction on Shrinking and Swelling Soils (Special Publication 11). Colorado Geological Survey; Wesley G. Holtz and Stephen S. Hart, October 1978.Published in 1978. We use it for soil behavior and landscaping distances, not for costs or current practice.
- RAP Bulletin 11: Slabjacking (ACI RAP-11). American Concrete Institute, Committee E706; primary author Fred R. Goodwin; copyright 2010.Covers slabs on ground (floors, pavements), not house foundations as a whole.